Light emitting semiconductors which emit in several regions of the visible spectrum, for example group III-V semiconductors such as aluminum gallium arsenide and gallium phosphide, have achieved commercial acceptance for various applications. However, for applications which require blue or green light, for example green to be used for traffic signal lights or blue for a component of a red-green-blue primary color combination to be used for white lighting, efficient semiconductor light emitters have been sought for shorter visible wavelengths. If such solid state light emitting sources were available at reasonable cost, many lighting applications could benefit from the reliability and low energy consumption that characterize semiconductor operation. Short wavelength devices also hold promise of providing increased storage capacity on storage media, due to the ability to obtain smaller spot sizes for writing and reading on the media.
Blue light-emitting diodes utilizing silicon carbide were developed during the early 1990's, but exhibited indirect bandgap luminance which limited the practicality of the devices. Zinc selenide, a group II-VI material, also produces blue light emission. Also, silicon carbide devices, as well as zinc selenide blue light emitting diodes, have been found to exhibit relatively short lifetimes that limit their usefulness.
A type of short wavelength light emitting devices that has direct energy bandgap, and has shown excellent promise, is based on group III-V nitride semiconductors, which include substances such as GaN, AlN, InN, AlInN, GaInN, AlGaN, AlInGaN, BAlN, BInN, BGaN, and BAlGaInN, among others. An example of a light emitting device of this type is set forth in European Patent Publication EP 0926744, which discloses a light emitting device that has an active region between an n-type layer of III-V nitride semiconductor and a p-type layer of III-V nitride semiconductor.
An electrical potential applied across the n and p layers of the diode structure causes generation of photons at the active region by recombination of holes and electrons. The wallplug efficiency of the light emitting diode (LED) structure is defined by the optical power emitted by the device per unit of electric power. To maximize efficiency, both the light generated per watt of drive power and the amount of light exiting from the LED in a useful direction are considered.
As noted in the referenced EP Patent Publication, a considerable effort has been expended in prior art approaches to maximize the light that is generated from the active region. The resistance of the p-type III-V nitride semiconductor layer is much higher than the resistance of the n-type III-V nitride semiconductor layer. The p-electrode junction with the p-type layer is inherently more resistive than the n-electrode junction with the n-type layer. To reduce the voltage drop across the p-electrode junction with p-type layer, the p-electrode is generally made much larger than the n-electrode. However, although this increase in size of the p-electrode may increase the amount of light available from the active region, it can decrease the fraction of light that exits the device, since much of the light must pass through the p-electrode. Accordingly, attempts were made to maximize the transmittance of the p-electrode.
In an embodiment disclosed in the above referenced EP Publication, the p-layer can be a layer of silver that is sufficiently thin to be transparent. It is noted that silver advantageously forms an ohmic contact at the p-type III-V nitride semiconductor layers. A metal bonding pad is deposited on the silver electrode. In another embodiment disclosed in the referenced EP Patent Publication, the silver layer is thick enough to reflect most of the light incident thereon, and light exits via the substrate. A fixation layer, such as another metal layer, which can be nickel, can be applied over and on the sides, of the silver layer, and prevents the diffusion of the metal (e.g. gold) of the contact bonding pad into the silver layer. The diffusion barrier layer is also stated to improve the stability underlying silver layer and improve the mechanical and electrical characteristics of the silver layer. As a result, it is stated that the substrate temperature during the vapor deposition step in which the silver layer is formed can be lowered and the vapor deposition speed increased.
The use of silver for at least the p-electrode in a III-V nitrides LED has advantages, but also suffers certain drawbacks and limitations. For example, the operational lifetime of such devices, before severe degradation of performance, has been found to be unacceptably short. It is among the objects of the present invention to address these drawbacks and limitations in existing III-V nitride LEDs.